The human light chain JC lambda locus is comprised of seven distinct segments, designated JC lambda 1, JC lambda 2, JC lambda 3, JC lambda 4, JC lambda 5, JC lambda 6, and JC lambda 7, Whereas three of these seven represent pseudogenes (Psi C lambda 4, Psi C lambda 5, and Psi C lambda 6), the JC lambda 1, JC lambda 2, and JC lambda 3 complexes are functional, as demonstrated by the finding of their protein products through sequence analyses of lambda-type Bence Jones proteins and light chains derived from monoclonal Igs. Although the JC lambda 7 segment also appears functional, as evidenced through analysis of lymphocyte-derived mRNA, heretofore no monoclonal JC lambda 7-containing lambda-chains have been identified. Serologically, two distinct isotypic markers, Meg and Oz, are associated, respectively, with JC lambda 1 and JC lambda 3 proteins, in contrast to JC lambda 2a components, which do not express these determinants and represent a third isotype. Although another serologic marker, Ke (Kern), considered a fourth isotype, has been assigned to the JC lambda 7 complex, this relationship has been questioned. We now report the primary structural features of a lambda-type Bence Jones protein that include the four distinctive residues encoded by the JC lambda 7 gene segment. This protein, obtained from a patient with multiple myeloma and designated MCP, represents the first example of such a molecule and provides definitive evidence that the JC lambda 7 gene complex is functional, Additionally, comparison of the C lambda sequences of Mcg(-)/Oz(-) Bence Jones proteins MCP and KERN supports the contention that the Ke-associated one-residue amino acid variation at position 152 reflects a C lambda 2 polymorphism and that yet another isotypic marker, provisionally designated Mcp, is encoded by the JC lambda 7 gene segment. Thus, we posit that there are four human JC lambda isotypes, Meg,Ke(-)Oz(-)/Ke(+)Oz(-), Ke(-)Oz(+), and Mcp, that represent, respectively, products of the JC lambda 1, JC lambda 2, JC lambda 3, and JC lambda 7 gene complexes.